Fig. 7

Tupanvirus soda lake biological features in a host (A. castellanii) and non-host (Tetrahymena sp.). a Expression of Tupanvirus 18S intronic sequence-copy 1 transcript, 12 h post-infection observed by fluorescence in situ hybridization (FISH) (red). Tupanvirus-induced shutdown of A. castellanii ribosomal RNA18S transcripts (green). Scale bars, 10 µm. b Tupanvirus, but not mimivirus, induces strong acidification of A. castellanii cytoplasm (9 h.p.i.), even in the presence of bafilomycin A1. Scale bars, 10 µm. c, d The silencing of the canonical autophagy protein Atg8-2 does not prevent rRNA shutdown induced by Tupanvirus infection. Error bars, standard deviation. e Electron microscopy of A. castellanii infected by Tupanvirus (2 h.p.i.), highlighting the degradation of the nucleolus, nuclear disorganization and the formation of ribosome-containing vesicles near nuclear membranes. Scale bar, 500 nm. Red arrow: single-membrane vesicles containing ribosomes; orange arrow: double-membraned vesicles containing ribosomes; asterisk: ribosomes wrapping by the external nuclear membrane. Right: electron microscopy of uninfected amoebae and APMV infected cell (8 h.p.i.) showing a mild nucleolar disorganization in the presence of the virus. Scale bars: 1 µm. f Haemacolour staining showing the nuclear degradation of A. castellanii induced by Tupanvirus infection (9 h.p.i.) compared with infection by mimivirus (APMV) and uninfected cells. Tupanvirus-infected cells are purplish because of cytoplasm acidification. Scale bars, 5 µm. Arrows: nucleus, when present; VF: viral factory. g Haemacolour staining showing the nuclear degradation in Tetrahymena sp. induced by Tupanvirus infection (4 days post-inoculation) compared with mimivirus (APMV)-inoculated cells and uninfected cells. Tupanvirus-infected cells present an atypical shape and intense vacuolization, and some cells lack a nucleus (asterisk). Arrows: nucleus under degradation. Scale bars, 5 µm. The experiments were performed 3 times independently, with two replicates each